1. Technical Field
The present invention relates to a charging device for use in an image forming apparatus such as an electrophotographic printer or copying machine or a facsimile, an image forming apparatus using the charging device and a charging controlling method.
2. Related Art
Heretofore, as a charging device used in an image forming apparatus such as an electrophotographic printer or copying machine or a facsimile, there has been used a contact type charging device wherein a charging roller is disposed in contact with the surface of a photoreceptor drum and an AC voltage or an AC voltage with a DC voltage superimposed thereon is applied to the charging roller to charge the drum surface uniformly to a predetermined potential.
As such a contact type charging device, there is known a charging device wherein, in order to suppress both the occurrence of unevenness in charging, i.e., non-uniform charging of the surface of a photoreceptor drum, and the creation of discharge products such as ozone and nitrogen oxides, a DC current flowing between the photoreceptor drum and a charging roller is detected and an AC voltage or current to be applied to the charging roller is controlled in accordance with the amount of variation of the detected DC current.
However, the current value of DC component in a bias voltage is used as information on variations in charging of the photoreceptor drum and the voltage of AC component is adjusted on the basis of the current value of DC component in the bias voltage, further, the bias control value applied during image formation to the contact charging member is determined by a value obtained by multiplying an alternating electric field value at a deviated point of an alternating electric field value−DC value from linearity by a predetermined ratio when the alternating electric field is gradually increased or decreased for a predetermined timing in a condition of not forming an image.
However, the current value of DC component flowing in the charging unit which is in contact with the image carrier is not only very small but also, according to measurement results obtained by the present inventors, as shown in FIG. 12, it is at the same level as the entire noise and there is no change in variation width (about 0.75 μA) of the current value of the DC component before and after saturation of the surface potential of the photoreceptor drum to a predetermined potential with a gradual increase of the AC voltage applied to the charging member.
Thus, according to the techniques, even if an attempt is made to control the AC voltage (alternating electric field) applied to the charging member on the basis of the current value of DC component in the bias voltage or the value detected by the DC current detecting unit, the current value of DC component in the bias voltage cannot be detected with a high accuracy, with the result that the charging potential of the image carrier such as a photoreceptor drum cannot be controlled with a high accuracy.